Syrian President Bashar al-Assad has suffered his most high-profile defection, with deputy oil minister Abdo Hussameddin quitting his post.

In an announcement posted on YouTube, Hussameddin said he was defecting and resigning, because he could no longer serve a "criminal regime."

"I am joining the revolution of the people who reject injustice and the brutal campaign of the regime, which is seeking to crush the people's demand for freedom and dignity," he said.

Hussameddin has served in the Syrian government for 33 years.

It comes as Senator John McCain called for the West to intervene in Syria on Thursday morning.

"Russia and the Iranians, they are shipping weapons in, [there was a] Washington Post article on Sunday that Iranians are physically there, so it’s already been internationalised and it’s not a fair fight.

"That’s why in my view it would require air power. It would require a coalition, not the United States going it alone. But if we don’t do this when we can prevent a massacre from taking place, then I think the United States will have abrogated its responsibilities," he told BBC Radio 4's Today programme.

Amos' spokesperson told the AFP news agency the city was "completely devastated".

Her visit is expected to result in the first outside assessment of conditions in an area of Homs city which has been the target of heavy military shelling and the scene of unquantifiable deaths, including those of Sunday Times journalist Marie Colvin and French photographer Remi Ochlik.

The Syrian government has not yet reacted to the defection.

The United Nations estimates more than 7,000 people have died since the Syrian government began to turn its fire on their own people.